Ceramic filters and commonly Ceramic Foam Filters (CFF) are currently available to purify liquid metal, such as disclosed in U.S. Pat. No. 3,893,917. Most often this involves the removal of solid inclusions from liquid metal, such as steel and aluminum. These solid inclusions can lead to physical defects in the final metal products if not removed prior to solidification.
In order to most efficiently use the filter media, the open porosity of the filter must be completely filled with liquid metal. Completely filing the filter with liquid metal improves the wetting of the surface of the filter media to facilitate the collection of the solid inclusions. The problem to be solved is that incomplete priming results in locally higher liquid velocities in the active parts of the filter, higher operational pressure drops or lower total liquid metal throughput, combined with lower collection efficiency for the solid inclusions.
A common practice is to place a ceramic foam filter with a gasket material into a filtering device or ‘bowl’, such that the metal height builds up over the filter and is forced by gravity into and through the filter medium. The inclusions are then removed by either deep or bed filtration mechanisms. The poor wetting characteristics of these ceramic filters and the need to remove the air contained within the pores, often leads to difficulties, particularly at the start of the filtration operation.
The significance of priming in filtration is disclosed in a number of Patents and Patent Applications, such as U.S. Pat. No. 4,872,908, where Enright, P. G. et al. describe the definition and role of priming in detail and also give specific efficiency data when removing 20 micron particles (between −13.4 and 54.8%) using LiMCA for 30 PPI filters The large range in filtration efficiency can be partly attributed to the impact of priming on filter performance. U.S. Pat. No. 4,081,371, Yarwood, J. C. et al. describe the need to remove gas bubbles from within the ceramic foam filter, and the roles of metallostatic head and filter angle on priming. Generally speaking higher total pressure (from metallostatic head or other means) improves priming efficiency. In U.S. patent application Ser. No. 09/867,144, Quackenbush, M. S., disclose a filter media, without the application of mechanical forces to encourage air bubble release, for the purpose of releasing trapped air bubbles to ensure an easier and more complete priming of the filter media.
In U.S. Pat. No. 7,666,248, Belley, L., et al. disclose a method using a vacuum system to generate an additional pressure gradient of about 6 kPa or about 25 cm of liquid aluminum head equivalent for the express purpose of increasing the effective priming pressure to ensure adequate priming for ceramic foam filters with a thickness from 2.5 to 7.6 cm and a low average pore or “window” size of 150-500 microns, which are typical of filters with 60 or more PPI. These filters otherwise require substantial metallostatic heads (vertical distance from trough bottom to filter top) to ensure adequate priming. Belley et al. also disclose that the typical range of priming heads for Ceramic Foam Filters is from about 20-80 cm. Higher values are associated with higher pore density and smaller window sizes, and are often impractical to implement at existing casting operations.
Filters are normally preheated to try to improve the flow of metal into the filter media and, hence, the priming efficiency for a fixed metal height over the filter. Difficulties are often encountered in obtaining uniform heating without localized overheating that can lead to thermal damage of the filter media. This makes it difficult to ensure that the entire filter area will be available to pass liquid metal. In U.S. Pat. No. 4,834,876, Walker, N. G. claims a process by which the non-conductive ceramic filter is rendered electrically conductive by the coating of the filter media particles with a conductive substance like nickel or by using an electrically conductive material, such as silicon carbide to construct the filter media. By passing a current through the media or by surrounding the filter with an induction coil to induce eddy currents, the media could be caused to self-heat due to the resistive (I2R) losses to ensure preheating and complete priming.
A process involving the use of a low frequency induction coil and Ceramic Foam Filter elements has been presented in U.S. Pat. No. 4,837,385 by Calogero, C. et al. In this process a number of different means were presented, whereby a crossed current and magnetic field could be created, which would generate Lorentz forces. Some of these methods involve the use of electrodes and a so-called ‘injection current’ which is undesirable as the electrodes are a potential source of contamination to the liquid metal being filtered. The theory behind the process disclosed by Calogero et al. was that the Lorentz forces would act preferentially on the metal and not the inclusions, thus causing migration of the inclusions and interception of the inclusions by the walls of the filter media. The impact of the magnetic field on the priming of the filter media was not disclosed. Furthermore, the mechanism described by Calogero depends on the absence of any significant curl or vorticity in the magnetic and Lorentz force fields. However, as disclosed in U.S. Pat. No. 4,909,836, vorticity is always present in these fields when a normal induction coil with a constant helical pitch is used as the source of crossed current and magnetic field. One aspect of the present invention uses an induction coil in order to avoid direct contact and contamination of the liquid metal. A standard constant pitch induction coil is used. The inventors are well aware of the vorticity in the magnetic and Lorentz forces produced via such an induction coil and have therefore designed the method to make maximum advantageous use of the vorticity, in order to press metal into the filter media to achieve a better degree of priming with a low metallostatic head.